High quality printing paper must have a number of physical properties. Two of the most important are a flat and smooth surface to facilitate printing in a press and gloss to produce a more attractive surface, particularly after printing. These properties can be obtained by a variety of techniques, such as coating the paper with pigments and binder and finishing it in one or more pressing operations.
One of the most common finishing operations employed in the manufacture of printing paper is supercalendering, in which paper is passed through a series of nips formed by steel rolls pressed against cotton filled rolls at very high pressures, typically at nip loads between 175 KN/M and 437.5 KN/M (1000 and 2500 pounds per lineal inch). This typically results in nip pressures of 13,780 KN/M.sup.2 to 27,560 KN/M.sup.2 (2000 to 4000 p.s.i.).
Traditional supercalender stacks are not externally heated, but heat is generated when the cotton filled rolls subjected to the extremely high pressures in the nip flex intermittently with each revolution. The nip temperatures in such supercalenders typically reach levels of about 71.degree. C. (160.degree. F.). Another important element in producing good results is having a high moisture content in the paper as it passes through the supercalender. Typically, the moisture content will be 7% to 9%, or higher, of the bone dry fiber weight. Flatness, smoothness and high gloss are obtained in supercalenders because of extreme compression and densification of the sheet. The densification undesirably results in reduced opacity and a blackening effect in overly moist portions.
Supercalenders commonly consist of a large number of rolls (9 to 14), alternating steel and resilient, in order to obtain the desired smoothness and gloss. In order to obtain smoothness on both sides it is necessary to run an even number of rolls and with two resilient rolls (so called "cushion rolls") running together midway in the stack to perform the necessary reversing of the side toward the steel rolls. This action is only partly successful at providing two smooth sides since the first side finished towards the steel is later deformed by the exposure to the resilient rolls.
Because of this shortcoming and the inherent mechanical problem associated with the "cushion roll" nip, many supercalenders operate today with an uneven number of nips and no "cushion roll" nip, which results in only one side being finished against the steel, and while gloss values may be manipulated to be close on the two sides, inevitably one side is noticeably rougher than the other,
Another form of finishing is machine calendering wherein the paper web is passed between two normally unheated steel rolls pressed together at high pressures. This process produces smoothness, but little gloss because of the absence of shear in the nip.
Another common finishing operation is gloss calendering, which uses heated finishing rolls to produce high gloss finishes on coated paper or board without the high pressure of supercalendering. The nip pressures for commercial machines are typically between about 87.5 to 175 KN/M (500 to 1000 pounds per lineal inch) of nip loading. This typically results in nip pressures of 6,890 KN/M.sup.2 to 13,780 KN/M.sup.2 (1000 to 2000 p.s.i.). The lower pressure causes less densification of the paper, and therefore, better opacity, while the higher temperature softens the coating and permits better gloss enhancement. However, the finishing effect is limited to the coating and the uppermost surface of the web. Thus, the surface of the sheet is not as smooth and flat as that produced in supercalendering and has generally been applied to coated board rather than high quality papers. As a result, gloss calendered sheets do not print as satisfactorily in a printing press as do supercalendered sheets.
In recent years, many modifications have been made to gloss calendering, machine calendering and supercalendering operations. Some supercalenders have been heated, primarily to improve the uniformity and control of the temperature. Typically, heated supercalenders reach nip temperatures of about 82.degree. C. (180.degree. F.). Temperatures of some machine calenders or supercalenders have been further increased in an attempt to allow a decrease in pressure to produce the same results. In spite of this modification of supercalendering in the direction of gloss calendering, the fundamental effects of the two processes have remained distinct. Supercalendering uniformly compacts the entire sheet to a high degree, thus flattening the surface fibers and all others, as well as producing gloss on the surface. In contrast, gloss calendering molds, flattens, and glosses the surface of the coating and, in the case of uncoated paper the top surface of the fibrous substrate, but compacts the remainder of the sheet much less than supercalendering.
Examples of gloss calendering are disclosed in U.S. Pat. Nos. 3,124,504; 3,124,480; 3,124,481; 3,190,212; and 3,254,593. These patents collectively describe apparatus capable of nip temperatures from below the boiling point of water to as high as 232.degree. C. (450.degree. F.) and nip pressures from 1,722 to 17,220 KN/M.sup.2 (250 to 2500 p.s.i.). U.S. Pat. No. 3,124,480 describes finishing steps designed to heat the coating on paper to a temperature which temporarily plasticizes at least the surface of the coating in contact with the hot drum. A form of supercalendering in which the rolls are heated to relatively high temperatures is disclosed in U.S. Pat. Nos. 3,442,685 and 3,451,331. These patents disclose a method and apparatus capable of producing high gloss on coated paper by heating at least one roll of a supercalender stack to a temperature between 82.degree. C. and 163.degree. C. (180.degree. F. and 325.degree. F.) to plasticize the coating.
The one parameter which has been found to be the most critical in gloss calendering and supercalendering has been the moisture content of the paper. High moisture improves the smoothing and glossing effects of both the coating and the paper substrate. Many developments in supercalendering and gloss calendering involve techniques for increasing the moisture in the web or at least in some portions of it before finishing.
Unfortunately, moisture is an undesirable control parameter. Small variations in moisture cause large variations in the finished properties of the paper. Also, it is undesirable to have more than about 3.5% to about 4.5% moisture in the finished sheet to avoid uneven reel building and sheet curl from later drying. This amount of moisture is a stable amount, and the sheet will not dry significantly below this level under ambient conditions. To have a finished product with the desired low moisture content and still have the desired high moisture content (e.g. 7% to 9%) to facilitate calendering, many heated calendering operations have increased the drum temperature to dry the moister webs.
Nonuniformity of moisture in the sheet can be even a bigger problem than too much moisture. By nonuniformity, it is meant that the moisture content at one place on the sheet is higher or lower than at other locations across the width of the sheet. The nonuniformity can also exist in the machine direction and the thickness of the sheet. Nonuniformity is most severe when calendering takes place immediately after coating, which is to say when the calender is in line with the coater. If coating is done in a separate operation from calendering, the moisture content of the coated paper has time to equalize throughout the web before calendering.
The above cited U.S. Pat. No. 3,124,504 is primarily concerned with very moist webs (up to 35% or 50% moisture) and includes the concept of drying the web while finishing it. Very high temperatures are employed for drying, but temperature above the boiling point of water are said to be needed only if the web is wetter than 5% to 8% of the bone dry weight. The web moisture content is also noted as being an important element in the process disclosed in above cited U.S. Pat. Nos. 3,442,685 and 3,451,331. The patents teach that it is best for the paper to have about 7% moisture content, and moisture can be added before the supercalender to improve the finishing effects. The addition of moisture before finishing is also described in above cited U.S. Pat. No. 3,124,482 to manufacture glazed uncoated paper. U.S. Pat. No. 2,214,641 also moistens the surface of the web before finishing. In U.S. Pat. No. 4,012,543, gloss calendering is undertaken immediately after coating before too much of the moisture is lost from the coating. In this disclosure, finishing is carried out at a web moisture content of 9% to 10% of the bone dry weight. In contrast, U.S. Pat. No. 3,268,354, takes special steps to dry the surface of the coating, but to maintain a wet interface between the coating and the fibrous web before gloss calendering. The web in this disclosure has a moisture content of at least 15% at the interface.